At present, an optical recording system has been widely prevalent as a system which writes information into a recording medium using light (light wave), or reads information from a recording medium. The optical recording system is well known as a typical recording and reproduction system employing CD, DVD, etc. as a recording medium. In these optical recording systems, it is required to record as much information as possible into a smaller medium, and efforts are now being made to raise the storage density per unit area of a recording medium.
As a method for enabling high density recording, recording information physically in a smaller recording bit was employed before. In this method, it is important to narrow down a light spot to as small a size as possible. However, it is considered to be difficult even to narrow down the spot to one half of the wavelength of light at the shortest. Therefore, an optical recording system conventionally employs light with a shorter wavelength, thus enabling it to attain a high density recording. However, a present DVD system already employs a blue laser with a sufficiently short wavelength, and it is difficult to shorten the wavelength of the DVD system further more because of restriction from an optical system etc. Then, a novel method for attaining a higher density recording is required to break through such a plateau.
There exists a novel method as one of the methods for performing higher density recording. The method does not reduce a recording bit size (physical bit size) but records information three-dimensionally into a medium to enhance a recording density with maintaining the recording bit size. Whereas a conventional recording system records information with recording bits two-dimensionally onto a planar medium, the new method records information additionally in a thickness direction of the medium to enhance a recording density. A system employing two-photon absorption is restricted to record information just by a light spot position thereof. This allows the system to move focusing position of the lens by changing a focusing position for bits in a depth direction of the recording medium, thereby allowing it to record information three-dimensionally. For example, if the system has 100 recording layers, the system allows it to increase the recording density by a factor of 100 without reducing a light spot size. As a result, the system can attain an actual high density recording without changing the wavelength.
However, this method is considered to have difficulty to increase the recording density unlimitedly, and is restricted to have hundreds of recording layers at most by the following reasons:    (a) The recording medium must be thicker in accordance with an increase in the number of the recording layers;    (b) When a write-in position is deep in the recording medium, light will attenuate at the position to make it difficult to acquire sufficient intensity of the light; and    (c) It is optically difficult to focus light precisely to such a deep position of the recording medium.
In a conventional recording system employing two-photon absorption, a laminating distance is reduced to give rise to a cross-talk between layers, thus making it necessary to separate the layers from each other by about several pm. There exists a limit that several hundreds of layers can be included in a 1 mm-thick medium at most.
Another method to attain a high density recording is proposed. The method employs near-field light to realize a microscopic recording bit of which size exceeds a diffraction limit of light. A conventional optical recording system employs a normal optical lens system for far-field light. However, it is difficult for the recording system to make a spot size of light less than a half of the wavelength of light, and the recording density thereof is limited by the wavelength of light. It is possible to employ near-field light instead of far-field light. Near-field light can exist near substances such as a lens and a pinhole, and does not propagate through a free space. When near-field light is employed, the optical recording system can narrow down a spot size of light to a size less than the wavelength of light, thereby allowing it to form a microscopic recording bit.
Recently, there has been known a method for forming a spot of near-field light smaller than a diffraction limit of light. The method employs a lens based on a surface plasmon. There are mainly two kinds of lenses based on a surface plasmon. One is known as a metal lens (“Laser Research”, Vol. 35, P. 572 (2007)). The lens is provided with a hole in a metal film. The hole is less than light wavelength in size, and is surrounded by concentric circular grooves. In this structure, the concentric circular grooves on the metal film are irradiated with light (light wave) to generate surface plasmons. The surface plasmons act so as to amplify light passing through the central hole of which size is less than the light wavelength. Therefore, the structure can remarkably increase an intensity of light, which can pass through the hole with the size less than the light wavelength, on the basis of surface plasmons, thereby allowing it to form a high-intensity spot of near-field light.
The other is a structure which is provided with concentric circular slits in a metal film, thereby forming a lens. The arrangement of the slits is optimized to allow it to form a near-field light spot in an area of near-field (Science 317, P. 927 (2007), Science 320, P. 511 (2008), Optics Express 13, P. 6815 (2005)).
The lens employing surface plasmons are referred to as the “plasmon lens” or “lens” simply in the description below. According to these plasmon lenses, it becomes possible to make a strong spot of near-field light in an area of the near field.
A spot size of near-field light is less than the diffraction limit depending on the light wavelength, and near-field light characteristically localizes near a pinhole or a lens. Therefore, near-field light cannot propagate long, and the intensity of near-field light attenuates exponentially. Also in an area far from the pinhole or lens, a portion of light emitted from a near-field light spot can propagate long as normal light. However, the portion of light lacks a component of near-field light, and can be focused by a normal lens on a small spot, whose size is about a half of the light wavelength, according to the diffraction limit of light. Therefore, it is considered to be difficult to focus light to form a light spot with a size of a half of the light wavelength in a deep area of the recording medium far from the pinhole or lens (by a distance of the light wavelength or more).
Recently, a complete lens (super lens) attracts much attention as a method for utilizing near-field light. The super lens is made of a material having a negative refractive index. The super lens is capable of propagating and focusing near-field light within the lens based on a material with negative electric permittivity and magnetic permeability. This lens allows it to focus light to form a fine spot, independently of the diffraction limit of light.
There are known several methods for making such a material with a negative refractive index. One of the methods is to implant microstructural bodies of metal or semiconductor in dielectrics, thus producing a metamaterial. Generally, the microstructural body is produced so as to be 10 to several 100 nm in size. The size of the microstructural body is considered to be preferably about a tenth of the light wavelength or shorter. The microstructural bodies of the metamaterial are produced so as to be sufficiently smaller than a light wavelength, thereby yielding an entirely averaged electric permittivity and magnetic permeability without a light wave blocked by each microstructural body.
It is revealed that a negative refractive-index material has an effect for amplifying near-field light, and a flat plate of the negative refractive-index material allows it to acquire a high resolution beyond a diffraction limit of light, thereby serving as a lens. In this way, a lens made of the negative refractive-index material attracts much attention as a lens capable of writing in microscopic bits beyond a diffraction limit of light.
However, the following is well known. That is, it is difficult to form a nano spot even by using a lens made of the negative refractive-index material in an area far from the lens by a distance of a light wavelength or more. The spot size exceeds a diffraction limit of light. This arises from a rapid attenuation of the near-field component when the near-field component is far from the lens by a distance more than a light wave length, even though the negative refractive-index material has an effect for amplifying near-field light. Particularly, when the negative refractive-index material has a dielectric loss, the attenuation of light becomes significant, and is difficult to solve essentially.
Even though near-field light localizing only near substances such as a lens or a pinhole allows it to make a spot size of light microscopic. Only a very thin surface of a recording medium can be irradiated with the near-field light. This leads to just a single layer recording, thereby giving rise to a limit on enhancement of the recording density for the optical recording.
A system which performs a three-dimensional recording using a negative refractive-index material is proposed (JP-A 2007-207395 (KOKAI)). JP-A 2007-207395 (KOKAI) discloses that employing the lens formed of the negative refractive-index material allows it to acquire a microscopic bit size of light less than a light wavelength and to realize a three-dimensional recording. However, even employing a super-resolution lens of the negative refractive-index material is considered to have a difficulty in focusing light to form a microscopic light spot in an area far from the lens with overcoming a light diffraction limit. This is because the super-resolution lens of the negative refractive-index material acquires a super resolution using an amplifying phenomenon of near-field light whereas the lens cannot actually propagate near-field light long as a result of a dielectric loss thereof. For this reason, it is impossible to acquire a sufficient focusing length by just using the negative refractive-index material for a lens, thereby making it difficult to apply the lens to a three-dimensional recording.
Furthermore, there is discovered a phenomenon that a light beam whose diameter is less than the wavelength thereof propagates in a certain kind of metamaterial without diffusing and changing the diameter thereof. The phenomenon is called a “canalization phenomenon” or a “self-focusing phenomenon”. That is, generally, even a light beam focused into a fine beam whose diameter is less than the wavelength thereof will expand naturally as a result of a diffraction phenomenon while propagating in a normal substance. However, even such a light beam focused finely can propagate directly without diffusing in the metamaterial, being called a canalization phenomenon. There are known several materials to cause such a phenomenon. It is disclosed that a multilayer film formed by laminating dielectric films and metal films or by laminating dielectric films and semiconductor films causes a canalization phenomenon in a direction perpendicular to the film surface of the multilayer film (Physical Review B, Vol. 74, P. 75103 (2006)). It is also disclosed that some kinds of photonic crystals cause a canalization phenomenon (IEEE Journal of selected topics in quantum electronics 8, P. 1246 (2002)). However, the reference does not suggest the application of the canalization phenomenon to information recordings. Moreover, in the canalization phenomenon, a light beam propagates in a straight line without diffusing or being focused. That is, it is impossible to selectively irradiate a specified area on a propagating line of the light beam with light, thereby making it substantially impossible to apply the canalization phenomenon to multilayer recordings.